I am specifically asking for your opinion, DrDave and Bob Jewett. Because you have put a lot of thought into deflection testing in the past and done quite a few experiments.
Would you consider this to be a valid test for measuring deflection? I have my doubts and I tend to believe you could achieve similar results with almost any shaft, LD, HD or CF. Maybe I'm wrong ...
https://www.youtube.com/watch?v=vWRuNjm5Vos
I see a technical issue with the test that might be a problem.
It looks like the distance of the shaft from the cushion is set by the thickness of his finger if his finger is tight against the nose of the cushion. It is good to control the left-right position of the cue stick. The problem though is that you want to control the other side of the shaft -- the one that hits the ball. (More exactly, it would be a point a little inside the other side of the shaft where the actual contact point on the ball is.)
Imagine a 2-inch diameter shaft. The bridge will force him to hit almost on the wrong side of the cue ball. That is an extreme case just to show you the direction of the influence. This points out that if the shafts are different diameters and the bridge does anchor the side of the shaft to the rail a systematic error is introduced.
When I demo squirt I show the shot I originally proposed to measure squirt: play a 90-degree cut shot with a ball frozen on the center of the far end rail by hitting rail first and using extreme inside english. I ask the student to estimate where my stick is actually pointed on a shot that makes the ball, such as for what looks like a full-ball aim or even aim on the wrong side of the ball. (I try to be near the miscue limit.)
Of course I should hit the ball at the same speed and with the same spin for each cue stick I do the demo with and maybe I come close to such control. For the purpose of demoing squirt and the fact that different cues have different amounts of squirt, that's not so important. In fact, I may have a similar error as the OP test for shafts of different diameter if I'm setting up the middle of the shaft in the same place -- I will have less side spin for thick shafts.